The electric motors generally used for the ventilation systems (air condition) of motor vehicles usually comprise a stator and a rotor. The commutator of the rotor is provided with contact plates resp. sections (generally made of machined copper) adapted to ensure an electric contact with brushes made of graphite (also called carbon brush). These brushes are attached to the stator.
To ensure a good electric contact between the commutator and the brushes it is well known that the brushes have to be pushed against the commutator by applying a sufficient pressure on them. If the contact is not adequate, the brushes can cause skips which in this way intermittently interrupt the electric contact. This results generally in sparks. By contrast if the contact is too strong the mechanical losses will increase as well as temperature and wear and tear. To this end the brushes are usually arranged in brush holders equipped with resilient means to maintain an electric contact between the brushes and the commutator.
In a well known manner the commutator has a rotation cylinder shape and supports a plurality of conductive plates resp. sections on its external periphery. The brushes are radially arranged around the commutator and comprise contact surfaces with the commutator sections. Brushes usually cover two or three commutator sections.
The brushes cover three commutator sections, if their rim width resp. polar or central angle is greater than 360°/P (with P=number of commutator sections). If the commutator comprises 12 sections, the brush covers three sections when the rim width angle has a value of more than 30°. This configuration has the disadvantage of generating noise, in particular during the initial working hours. This basically involves the incertainty of commutation (random contact point between the brush and the commutator section). After some working hours a patina constituted of a deposit generally containing metal (and its oxides), carbon and water will form on the commutator surface. Said patina tends to increase the motor function noise. However, this patina will form only after some motor working hours. Therefore this type of motor generates too much noise during its initial working hours, which is not compatible with its arrangement in a passenger compartment of a motor vehicle.
The brushes cover two commutator sections if their rim width angle resp. polar angle is in the range of 360°/P (with P=number of commutator sections). Said brushes have the advantage of generating less noise during the initial working hours, but present the characteristic of increasing the temperature level. To dissipate the heat the friction area with the commutator has been extended, but this results in a modification of the required space of the brushes.